This invention relates generally to fiberization dies for applying hot melt adhesives to a substrate or for producing nonwovens. In one aspect the invention relates to a modular die provided with an internal rotary positive displacement pump. In another aspect, the invention relates to a segmented die assembly comprising a plurality of separate die units, each unit including a manifold segment and a die module and recirculation module mounted thereon.
The deposition of hot melt adhesives onto substrates by fiberization dies has been used in a variety of applications including diapers, sanitary napkins, surgical drapes, and the like. This technology has evolved from the application of linear beads such as that disclosed in U.S. Pat. No. 4,687,137, to air-assisted deposition such as that disclosed in U.S. Pat. No. 4,891,249, to spiral deposition such as that disclosed in U.S. Pat. Nos. 4,949,668 and 4,983,109. More recently, meltblowing dies have been adapted for the application of hot melt adhesives (see U.S. Pat. No. 5,145,689). As the term suggests, "fiberization" refers to a process wherein a thermoplastic melt is extruded into and set into fibers.
Modular dies have been developed to provide the user with flexibility in selecting the effective length of the fiberization die. For short die lengths only a few modules need be mounted on a manifold block. (See U.S. Pat. No. 5,618,566). Longer dies can be achieved by adding more modules to the manifold. U.S. Pat. No. 5,728,219 teaches that the modules may be provided with different types of die tips or nozzles to permit the selection of not only the die length but the deposition pattern.
U.S. Pat. No. 5,236,641 discloses a metering die which comprises a plurality of metering pumps which feed polymer to individual regions of a single elongated die tip. The tip is mounted on a single polymer manifold which has a plurality of side-by-side flow channels which feed a predetermined number of orifices of the tip. Each pump supplies polymer to a single channel. The pumps may be turned on or off so that polymer flow may be discontinued to some of the orifices of the integral elongate tip. In this design the length of the die is not variable because the manifold and die tip are of fixed length and are not formed from individual segments.
At the present, the most commonly used adhesive fiberization dies are intermittently operated air-assisted dies. These include meltblowing dies, spiral nozzles, and spray nozzles.
Meltblowing is a process in which high velocity hot air (normally referred to as "primary air" or "process air") is used to blow molten fibers or filaments extruded from a die onto a collector to form a nonwoven web or onto a substrate to form an adhesive pattern, a coating, or composite. The terms "primary air" and "process air" are used interchangeably herein. The process employs a die provided with (a) a plurality of openings (e.g. orifices) formed in the apex of a triangular shaped die tip and (b) flanking air plates which define converging air passages. As extruded rows of the polymer melt emerge from the openings as filaments, the converging high velocity hot air from the air passages contacts the filaments and by drag forces stretches and draws them down forming microsized filaments. In some meltblowing dies, the openings are in the form of slots. In either design, the die tips are adapted to form a row of filaments which upon contact with the converging sheets of hot air are carried to and deposited on a collector or a substrate in a random pattern.
Meltblowing technology was originally developed for producing nonwoven fabrics but recently has been utilized in the meltblowing of adhesives onto substrates. Meltblown filaments may be continuous or discontinuous.
Another type of die head is a spiral spray nozzle. Spiral spray nozzles, such as those described in U.S. Pat. Nos. 4,949,668 and 5,102,484, operate on the principle of a thermoplastic adhesive filament being extruded through a nozzle while a plurality of hot air jets are angularly directed onto the extruded filament to impart a circular or spiral motion thereto. The filaments thus form an expanding swirling cone shape pattern while moving from the extrusion nozzle to the substrate. As the substrate moves with respect to the nozzle, a circular or spiral or helical bead is continuously deposited on the substrate, each circular cycle being displaced from the previous cycle by a small amount in the direction of substrate movement. The meltblowing die tips offer superior coverage whereas the spiral nozzles provide better edge control.
Other fiberization dies include the older non-air-assisted bead nozzles such as bead nozzles and coating nozzles.